Here's the best way, if you can't use the Pokedex to spot it's location:
-Beat elite 4 (1st time)
-Catch 60 types of pokemon and talk to professor oak (He gives you the ability to find johto's pokemon...get the amulet coin while you're at it)
-complete the sevii islands crap (fast way to make money)
-On 5 island, use the VS seeker to battle the two rich girls at resort gorgeous until you have enough for about 100 max repels. (this should only take 4 battles total, as you get 19.5k - 20k per battle)
-Beat elite 4 (2nd time, mostly for kicks and good measure. I'm not sure if it has any effect on the runners or not, but it's better safe than wasting 3 hours)
-Buy 100 max repels and fly to celadon city, and bike right (east) until you hit the patch of grass on route 7.
-Now, with a pokemon that's somewhere between level 22 and 39 in the lead, use a max repel, and bike in the grass until the repel runs out. At this point leave the grass and enter the building with the guard in it, then exit again. Apply another max repel and repeat until the runner shows up.
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Basically you just need acess to the johto pokedex and a lot of max repels. Route 7 only has one patch of grass, so if he's there, you'll find him. The runner switches position every time you enter a new area, including buildings. So just keep running from the grass, to the building, and back again.
This may take a LOT of time. I used a total of 120-ish max repels and about 2 hours to find mine without being able to see where it was. SOme people say they're found it in under 20 max repels under the same circumstances. (Some people are just lucky)
My biggest suggestion is don't use fly, or turn off your game, otherwise it will completely randomize it's location, as opposed to moving by only one route at a time as normal. Switching patches will only complicate the issue. Just stick to the same place and they'll turn up eventually.